DE-A-197 35 208 discloses a switched-mode power supply unit which has on the primary side switching means which bring about safe switching off of the switching transistor in the event of a secondary short-circuit. The occurrence of a short-circuit is in this case detected at a primary winding of a transformer.
U.S. Pat. No. 6,567,252 discloses a protection circuit which is arranged on the secondary side and in which a number of output voltages to be monitored of a switched-mode power supply unit are connected to a switching stage via a diode and resistor network. The switching stage is connected to a control loop of the switched-mode power supply unit and switches off the switched-mode power supply unit if a short-circuit occurs.
Protection circuits of this type, which monitor a number of output voltages, are used in particular in switched-mode power supply units based on the isolating transformer principle. These produce a large number of stabilized output voltages for circuits that are used for example in television sets, video recorders and satellite receivers. Devices of this type usually have a normal mode, in which all circuits are activated, and a so-called standby mode, in which the least possible power is used and in which usually only circuits that are required to switch the device over to the normal mode by means of a remote control are active. The power supply units used in these devices therefore have output voltages which are only present in the normal mode and output voltages which are present both in the normal mode and in the standby mode.